Screens is a Screen Sharing (VNC) and Remote Login (SSH) client application for macOS and iOS. Screens establishes unencrypted and encrypted connections with remote computers to display the remote desktop on the local display. Screens gives you remote control over computers on your local network or virtually anywhere in the world via the Internet. Screens provides all the client functionality needed; neither Screens Connect nor Screens Express are required to use Screens.
Screens Connect does none of this. Screens Connect is a freely available and optional companion application that enhances your Screens experience by assisting with network configuration and automatically maintaining Screens' Nearby Computers and Screens Connect lists in the Screens Library. With Screens Connect, you don't have to worry so much about IP addresses, open ports, or routing issues. Screens Connect is not required to use Screens.
Screens Express provides a temporary point of contact for Screens, similarly to Screens Connect, but designed for one-off connections, such as when helping a clueless colleague solve a problem with his computer. How does it work in practice? Take a look. Screens Express is not required to use Screens.